1. Field of the Invention
The present invention relates to improvements in through-air dryer (TAD) fabrics, in particular for use on tissue-making machines.
2. Description of the Related Art
A typical tissue-making machine includes a forming section, such as a C-former, wherein the fibrous slurry is injected between two forming wires with the web forming on the outer wire. The TAD section is positioned between the forming section and yankee dryer cylinder, and typically includes at least one large diameter TAD roll or cylinder with a perforated or honeycomb working surface, hot air being passed through the surface of the roll and the TAD fabric with the paper web thereon.
In the art, TAD fabrics are usually made of PET monofilament that has been modified for high temperature applications. TAD fabrics are coarser than a forming fabric, but finer than a standard dryer fabric. Typical dryer fabrics have over 100% warp cover, including two or more layers of warp yarns, which overlap so that all fabric areas are covered, in some regions by a double layer of yarn. TAD fabrics are much more open, with typical warp coverage of about 60%.
A TAD fabric needs sufficient open area to allow air to pass through, once it has passed through the paper web, so as to promote efficient drying. The fabric must also have a high sheet contact area on the face side of the fabric to ensure successful sheet transfer to the yankee cylinder from the TAD. A standard TAD fabric directly after weaving has a contact area of about 6–12%. For effective sheet transfer, a contact area of 20–30% with a target of 25% is required. Up to the present, this has been achieved by sanding the fabric after weaving, see for example U.S. Pat. No. 3,573,164 and GB-A-2104565.
A number of problems are created through the use of sanding. One problem which arises in sanding is that some or all of the top half of the circular cross-sectional yarn is removed which results in significant or severe weakening of the yarns, in turn rendering the whole fabric inherently weak, or less stable.
The fabrics are made using highly drawn and as such highly tenacious polyester warp yarns. These yarns have high crystallinities and high molecular orientations as a result of the extrusion process. When such yarns are sanded in the lengthwise direction, the surface of the yarns become very rough. Following installation of the fabric, when high pressure showers are in use, the yarns tend to fibrillate, small pieces of yarn peeling away and often ending up in the cross-over point of the weave. This process is accelerated by temperature, so making the fabric edges particularly vulnerable.
There have been difficulties with the sanding process itself in that preferential sanding of the fabric occurs, whereby the new sand paper gradually becomes worn down, so loosing efficiency, until it again needs to be replaced. After sanding there have also been difficulties in removing the abraded dust from the fabric, due to static charges, due to which the dust often gets into the yarn cross-over points which can cause problems from the outset.
Additional problems are that sanding is a notoriously slow and thus costly process and also has serious health risks associated with it due to the dry dusty environment in which it must be carried out.
The use of flat yarns in papermachine fabrics has been proposed, notably as summarized in the introductory part of U.S. Pat. No. 5,407,737 (Halterbeck) which is concerned with a dryer screen including flat yarns obtained by flattening tubular yarns of a circular cross-section. U.S. Pat. No. 5,449,026 (Lee) proposes a multilayer dryer fabric with several layers of machine direction flat yarns woven therein. The use of high aspect ratio yarns of over 3:1 is discussed, and the drawings illustrate tape-like yarns with an aspect ratio of 6:1. The aspect ratio is the ratio of the width to the thickness of the yarn, expressed as W/E. Such wide flat yarns provide good yarn cover in the fabric and are thus useful to restrict air and water permeability, and they also produce a low weave thickness as compared with round yarns giving the same cover. However during weaving care must be taken to avoid imparting any degree of twist to the flat yarns as a twist in such a flat yarn occupies a thickness related to the width of the yarn, and produces a pronounced irregularity in the weave.
What is needed in the art is a through-air dryer fabric with flat warp yarns and/or flat weft yarns which does not require sanding.